HALL EFFECT IN NIOBIUM DOPED YBa_2Cu_3O_(7-y) SUPERCONDUCTOR

YBa_2Cu_(3-x)Nb_xO_(7-y) superconductors have been prepared. XRD shows that the system remains orthorhombic for all composition. Substitution of Nb~(5+) for Cu~(2+) occurs in the Cu(2) sites. The introduction of the Nb~(5+) ions produces some extra free-electrons. These electrons recombine with the carriers of the system. It makes the mobility and the Hall number of YBa_2Cu_(3-x)Nb_xO_(7-y) decrease and also result in a depression in Tc.     

高T_c LaBa_2Cu_3O_(7-δ)超导相的制备和结构研究

我国蕴藏着丰富的稀土元素,利用镧代替钇,研究高T_c超导体具有一定的经济意义。但镧和钡的原子半径相近而比钇大得多,这给制备工作带来了困难。国内曾有几个单位制备出了T_c80K的超导相,但都难以重复。日本A.Maeda等人合成了T_c80K的正交LaBa_2Cu_3O_(7-δ)超导相,测定了晶体结构和相转变温度。最近我们也开展了对这个体系的研究,本文将给出隐定的高T_cLaBa_2Cu_3O_(7-δ)超导相的制备方法、相变温度、晶体结构以及超导性能与正交性的关系。     

超导相YBa_2Cu_3O_(7-d)的制备与结构研究

本文按计量式Y_(0.334)Ba_(0.666)CuO_(3-y)制备和得到了题目超导物相的样品。实验中使用柠檬酸为复合剂显著改善了金属氧化物的分散性和超导物相的均匀性。我们发现制备所得样品中,偏离四方晶格愈甚者,其超导性能愈佳。由X射线粉晶衍射数据测定了晶体结构,确认了金属离子的化学计量比和给出了氧原子的位置。正交或假四方晶胞的晶胞参数为a=3.893,b=3.813和c=11.867。晶胞由铜原子占据顶点位置的三个钙钛矿型立方体所组成,二个钡原子居于上层和底层的两个体心位置,而钇原子则占据居中立方体的体心位置。氧原子均占据诸立方体的稜心位置,但001/2处为空位,而01/20处的空位则导致晶格对称性由四方畸变为正交,这一畸变对超导相有深远的影响。钇和钡原子的配位多面体系由立方-八面体所派生的八配位和十配位的多面体,两者分别与移去晶胞中001/2和01/20处的氧原子后的氧多面体相对应。根据键价理论,以Y—O,Ba—O和Cu—O键长为基础,我们合理地论述了,即使在001/2及01/20处氧缺位的情况下,1/200处氧原子的占有率亦将低于1,与晶胞对应的该物相化学式为YBa_2Cu_bO_(7-d),0.2相似文献   

REBa_2Cu_3O_(7-δ)超导体系列中稀土离子置换对XPS影响的研究

测量了REBa_2Cu_3O_7-δ(RE=Sm、Eu、Gd、Dy、Ho、Y)单相氧化物超导体各元素的室温XPS特征峰,低温下YBa_2Cu_3O_7-δ的XPS,R-T曲线和晶格参数。对所有化合物,从XPS上没有直接观察到Cu~(3+)。室温研究结果表明,E_f处的强度很低,具有磁性稀土离子的置换对超导体E_f处的强度影响不大。YBa_2Cu_3O_7-δ的低温XPS测量进一步表明,对超导体电子结构起重要作用的是氧配位体上的空穴。     

EPR SPECTRA OF THE HIGH Tc SUPERCONDUCTOR YBa_2Cu_3O_7

<正> INTRODUCTION. The research on a series of high T_c Y-Ba-Cu-O superconductors since its dxscovery has been found interesting. Preparations and structural studies concerning this field, as well as some spectroscopic work on the study of the superconductive mechanism have been reported. In this communication, we report the EPR studies of the superconductor YBa_2Cu_3O_7 both at room temperature and at liquid N_2 temperature.     

Influence of vanadium doping on the behavior of YBa_2Cu_3O_7-y

YBa2Cu3-xVxO7-y(x=0, 0.1, 0.2, 0.3, 0.4, 0.5 and 0.6) superconductors have been prepared. X-ray diffraction shows that the system remains orthorhombic for all compositions studied, but for x > 0.4 V2O5 was detected as an impurity phase. Substitution of V5+ for Cu2+ occurs in the Cu(2) sites on the Cu(2)-O planes. The introduction of the high valence element, vanadium, produces the extra free-electrons. These electrons recombine with the positive carrier of the system. It makes depression of the mobility and the Hall number of YBa2Cu3-xVxO7-v and also results in a depression of TC.     

高T_c YBa_2Cu_3O_(7-δ)起导相生成的机理

自M.K.Wu等发现高T_cYBa_2Cu_3O_(7-δ)超导体以来,人们从各个不同的角度对这个体系进行了大量的研究工作,对它的制备、结构、性能及应用都有了一定的认识。我们在制备高T_cYBa_2Cu_3O_(7-δ)超导体时,发现在摄氏900℃以上的高温,烧结温度仅差十几度足以引起超导性能的明显差别,这是令人难以理解的。为此有必要对达个体系在烧结过程中的相变、氧含量变化及各相的结构作进一步的研究,以求弄清YBa_2Cu_3O_(7-δ)超导相生成的机理。     

熔融织构生长(MTG)法制备的YBa_2Cu_3O_(7-x)超导晶体点阵缺陷的同步辐射形貌研究

自Jin等人首次采用熔融法合成类单晶显微结构的体材以来,许多研究者已获得具有高度C轴取向的类单品或高质量的单品。我们用同样方法也制得了在77K、2T磁场下,J_c为2.38×10~4A/cm~2的YBa_2Cu_3O_(7-x)超导晶体,并用高压透射电镜观察证实了高密度的位错、位错环以及弥散分布的211相,CuO粒子可能是主要的磁通钉扎中心,从而大大地     

高Tc LaBa2Cu3O7-δ超导相的制备和结构研究

A stable orthorhombic superconduting phase LaBa_2Cu_3O_(7-δ), with T_c 82K and Meissner effect 13.6%, has been prepared by citric acid complexation with nitrates. It is determined to be an orderly three layer Ba-La-Ba, oxygen-deficient perovskite structure by X-ray powder diffraction and atomic valence calculation. Through high temperature XRD and DTA-TG study,the phase transition of orthorhombic into tetragonal structure is found to be reversible, and occurs at 450 ℃, not the value 350 ℃ proposed by A. Maeda et al. The superconduting properties are improved as the responsible orthorhombic structure deviates more from tetragonality.     

高Tc超导体YBa2CU3Ox中铜的价态研究

应用多种光致发光光谱技术测定了高Tc超导体YBa_2Cu_3O_x的荧光光谱,对谱带的比较研究和指认表明,在这种样品中同时存在着Cu~ 、Cu~(2 )和Cu~(3 )。     

HYDROGENATION OF CO_2 OVER A YBa_2Cu_3O_(6～7) CATALYST

研究了ＹＢａＣｕ３Ｏ６～７超导催化剂上ＣＯ２的加氢制醇反应。考察了温度、压力和空速等条件对催化剂反应性能的影响。反应的主要产物是甲醇、ＣＯ和少量甲醚。利用ＸＰＳ、ＸＲＤ和ＡＦＭ等技术对催化剂的结构、铜的存在状态和反应活性位进行表征发现，在反应过程中，ＹＢａ２Ｃｕ３Ｏ６～７由ｏｒｔｈｏｍｂｉｃ相转变为ｔｅｔｒａｇｏｎａｌ相。反应活性位可能是Ｃｕ（Ｉ）物种。反应后催化剂颗粒的分散程度明显提高     

Nd_(1 x)Ba_(2-x)Cu_3O_(7-δ)超导体的热处理

通过对Nd1 xBa2 -xCu3O7-δ的两种样品 (烧结样品和区域熔炼样品 )的XRD谱图分析 ,研究了高温氩气氛下热处理对样品中Nd对Ba的取代值x的影响 ,同时比较了不同样品的Tc 和Jc 值。研究表明 ,氩气下 95 0℃热处理可以减少Nd对Ba的取代 ,Tc 和Jc 都有不同程度的提高。对不同固溶度x值的Nd1 xBa2 -xCu3O7-δ进行吸氧热处理 ,样品在热处理前后分别作XRD分析。从XRD谱图可以看出 :当x <0 4时 ,样品经吸氧后可以从四方相转变为正交相 ,而当x >0 4时 ,样品虽经长时间吸氧也不能使其从四方相转变为正交相     

超导相(Bi_(1.7)Pb_(0.2)In_(0.1))Sr_2Ca_2Cu_3O_(10)的制备和晶体结构研究

Bi-Sr-Ca-Cu-O体系的掺杂可以改善超导性能和消除堆垛层错。由于铋氧双层的作用还未能很好地了解,我们合成X_2Sr_2Ca_2Cu_3O_(10)的纯相,[X为(Bi_(0.85)Ph_(0.1)In_(0.05)]以确定其是(XOXO)还是(XO_2X),以及如何影响晶体结构,超导性质和生成机理。合成和鉴定CaCO_3、SrO、CuO的混合物于900℃烧结20hr得到Sr_2Ca_2Cu_3O_7,然后与Bi_2O_3、PbO按摩尔比1∶0.87∶0.3混匀,压片,于870℃加热20hr,重新研磨,在880℃加热30hr,再加0.6%mol In_2O_3,于890℃烧结20hr,然后在空气中淬火,样品表面部分微熔。     

INFLUENCE OF SILVER-DOPING ON THE SUPERCONDUCTIVITY FOR YBa_2Cu_30_(7-y)

The effect of substituting silver for barium on the structure and superconducting properties of YBa_2Cu_3O_(7-y) has been studied. The structure of the silver-doping system (YBa_(2-x)Ag_xCu_3O_(7-y)) with x<0.3 is orthorhombic. For x>0.3, YBCO, Y_2BaCuO_5 phase and metallic silver ro-exist in the system. The superconducting transition temperature T (R=0) decrease with increasing silver content. However, the critic current density J_c is found to be enhanced from 56.6 A/cm~2 to 195 A/cm~2     

PREPARATIVE AND STRUCTURAL STUDIES ON THE SUPERCONDUCTING PHASE YBa_2Cu_3O_(7-d)

Reasonably homogeneous samples were obtained by following the formula Y_(0.334)Ba_(0.666)CuO_(3-y) andusing citric acid as a complex-forming agent to improve the dispersion of the metal oxides. We havealso found that the variant condition in preparative procedure leads to variant deviation from tetrago-nality and that more pronounced deviation gives better superconducting properties. The crystal structure determined from X-ray powder diffraction diagrams confirms the stoichiom-etry of the metal ions and gives the positions of the oxygen atoms. The orthorhombic or pseudo-tetragonal unit cell with a=3.893, b= 3.813 and c= 11.687 A contains YBa_2Cu_3O_7. It is composed ofthree perovskite cubes with three copper atoms at their corners, the two barium atoms at the centers ofthe top and bottom cubes, and the yttrium at the center of the unit cell. The oxygen atoms sit on centersof cube edges. The positions (0 0 1/2) are vacant. The vacancies represented by (0(1/2)0) cause the ar-rangement of atoms to lose tetragonal symmetry, and have far-reaching effect for this phase. The yttriumand barium atoms are coordinated in a cuboctahedron respectively with four equatoria oxygen atoms at(0 0 1/2) and two at (0 1/2 0) removed. In the light of bond valence theory we can infer from the in-teratomic distances Y-O, Ba-O and Cu-O that the occupancy of the oxygen atoms at (1/2 0 0) is wellbelow unity while the positions (0 0 1/2) are vacant, the formula of the phase is YBa_2Cu_3O_(7-d), thefraction of Cu(Ⅱ) and (Ⅲ) for the set of copper at (0 0 0) is respectively d/0.5 and (1-d/0.5), and0.2相似文献   

潮湿空气中YBa_2Cu_3O_(7-x)超导体表面分解的SIMS研究

本实验利用SIMS研究了YBa_2Cu_3O_(7-X)超导体长期暴露在潮湿空气中的表面分解问题。结果表明,YBC超导体与湿空气中H_2O和CO_2发生化学反应,则使表层分解成BaCO_3、Y_2O_3和CuO等物质。     

YBa_2Cu_(3-x)Fe_xO_(7-δ)陶瓷样品的超声特性研究

本文研究了YBa_2Cu_(3-x)Fe_xO_(7-δ)(0≤x≤0.4)高温氧化物多晶超导体系的电阻,超声声速和超声衰减随温度的变化特性。对于超导样品,声速-温度曲线出现三次明显异常,首先是在超导转变温度附近声速呈现一极小值,表明样品中出现声子软化现象,其次分别在170K和240K附近声速发生两次大的突变,同时超声衰减曲线也相应地出现两个峰,而不超导的样品(x=0.40)只有170K和240K两处出现超声异常,而低温下无明显的声子软化现象,这说明,高温超导相变的发生会导致样品的弹性性质发生变化,而240K处体系发生的相变同高温超导电性无必然的联系,文中还初步讨论了170K超声异常的起因。     

SrFe_(0.6)Cu_(0.3)Ti_(0.1)O_(3-δ)混合导体透氧材料的稳定性

利用柠檬酸络合法制备了SrFe(Cu,Ti)O_(3-δ)混合导体透氧材料. 采用XRD、O_2-TPD、H_2-TPR、SEM等测试技术考察了材料的稳定性. 结果表明,SrFe_(0.7)Cu_(0.3)O_(3-δ)在低氧压下会发生相分解,产生SrCuO__2杂相,而掺杂Ti后的SrFe_(0.6)Cu_(0.3)Ti_(0.1)O_(3-δ)在低氧压下保持单一的钙钛矿结构. H_2-TPR和O_2-TPD的测试表明,Ti~(4+)的掺杂提高了材料的氧脱附起始温度和其它金属离子的还原温度. SrFe_(0.6)Cu_(0.3)Ti_(0.1)O_(3-δ)膜在透氧过程中,会有Cu~(2+)和Sr~(2+)从钙钛矿结构中析出,在原晶粒边界形成新的小晶粒,但这种轻微的组分偏析没有影响到材料的透氧量,此透氧膜在66 h的操作过程中显示了良好的稳定性.     

YBa_2Cu_3O_7的化学键性质和穆斯堡尔谱的研究

利用电介质的平均能带模型计算了YBa2 Cu3O7的化学键参数 ,得到Cu(1)—O键的平均共价性为 0 40 6,Cu(2 )—O键的平均共价性为0 2 76。应用化学环境因子计算了Fe :Y 12 3的穆斯堡尔同质异能位移 ,确定了5 7Fe在Y 12 3中的价态和占位情况。由电声子强度参数与共价性的关系说明了超导相中Cu(2 )离子的重要性